Astronomers using the National Science Foundation’s Very Large
Array (VLA) radio telescope are taking advantage of a
once-in-a-lifetime opportunity to watch an old star suddenly
stir back into new activity after coming to the end of its
normal life. Their surprising results have forced them to
change their ideas of how such an old, white dwarf star
can re-ignite its nuclear furnace for one final blast of
energy.

Computer simulations had predicted a series of events that
would follow such a re-ignition of fusion reactions, but
the star didn’t follow the script — events moved 100 times
more quickly than the simulations predicted.

“We’ve now produced a new theoretical model of how this process
works, and the VLA observations have provided the first
evidence supporting our new model,” said Albert Zijlstra, of
the University of Manchester in the United Kingdom. Zijlstra
and his colleagues presented their findings in the April 8
issue of the journal Science.

The astronomers studied a star known as V4334 Sgr, in the
constellation Sagittarius. It is better known as “Sakurai’s
Object,” after Japanese amateur astronomer Yukio Sakurai, who
discovered it on February 20, 1996, when it suddenly burst
into new brightness. At first, astronomers thought the outburst
was a common nova explosion, but further study showed that
Sakurai’s Object was anything but common.

The star is an old white dwarf that had run out of hydrogen
fuel for nuclear fusion reactions in its core. Astronomers
believe that some such stars can undergo a final burst of
fusion in a shell of helium that surrounds a core of heavier
nuclei such as carbon and oxygen. However, the outburst of
Sakurai’s Object is the first such blast seen in modern times.
Stellar outbursts observed in 1670 and 1918 may have been
caused by the same phenomenon.

Astronomers expect the Sun to become a white dwarf in
about five billion years. A white dwarf is a dense core
left after a star’s normal, fusion-powered life has ended.
A teaspoon of white dwarf material would weigh about 10
tons. White dwarfs can have masses up to 1.4 times that
of the Sun; larger stars collapse at the end of their
lives into even-denser neutron stars or black holes.

Computer simulations indicated that heat-spurred convection
(or “boiling”) would bring hydrogen from the star’s outer
envelope down into the helium shell, driving a brief flash
of new nuclear fusion. This would cause a sudden increase
in brightness. The original computer models suggested a
sequence of observable events that would occur over a few
hundred years.

“Sakurai’s object went through the first phases of this
sequence in just a few years — 100 times faster than we
expected — so we had to revise our models,” Zijlstra
said.

The revised models predicted that the star should rapidly
reheat and begin to ionize gases in its surrounding region.
“This is what we now see in our latest VLA observations,”
Zijlstra said.

“It’s important to understand this process. Sakurai’s Object
has ejected a large amount of the carbon from its innner core
into space, both in the form of gas and dust grains. These
will find their way into regions of space where new stars form,
and the dust grains may become incorporated in new planets.
Some carbon grains found in a meteorite show isotope ratios
identical to those found in Sakurai’s Object, and we think
they may have come from such an event. Our results suggest
this source for cosmic carbon may be far more important than
we suspected before,” Zijlstra added.

The scientists continue to observe Sakurai’s Object to take
advantage of the rare opportunity to learn about the process
of re-ignition. They are making new VLA observations just
this month. Their new models predict that the star will heat
very quickly, then slowly cool again, cooling back to its
current temperature about the year 2200. They think there
will be one more reheating episode before it starts its
final cooling to a stellar cinder.

Zijlstra worked with Marcin Hajduk of the University of
Manchester and Nikolaus Copernicus University, Torun, Poland;
Falk Herwig of Los Alamos National Laboratory; Peter A.M. van
Hoof of Queen’s University in Belfast and the Royal Observatory
of Belgium; Florian Kerber of the European Southern Observatory
in Germany; Stefan Kimeswenger of the University of Innsbruck,
Austria; Don Pollacco of Queen’s University in Belfast; Aneurin
Evans of Keele University in Staffordshire, UK; Jose Lopez of
the National Autonomous University of Mexico in Ensenada; Myfanwe
Bryce of Jodrell Bank Observatory in the UK; Stewart P.S. Eyres
of the University of Central Lancashire in the UK; and Mikako
Matsuura of the University of Manchester.

The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement
by Associated Universities, Inc.